1 files changed, 93 insertions, 0 deletions
diff --git a/src/home/time.asm b/src/home/time.asm
new file mode 100644
index 0000000..83100c7
--- /dev/null
+++ b/src/home/time.asm
@@ -0,0 +1,93 @@
+; timer interrupt handler
+TimerHandler: ; 01e6 (0:01e6)
+	push af
+	push hl
+	push de
+	push bc
+	ei
+	call SerialTimerHandler
+	; only trigger every fourth interrupt ≈ 60.24 Hz
+	ld hl, wTimerCounter
+	ld a, [hl]
+	inc [hl]
+	and $3
+	jr nz, .done
+	; increment the 60-60-60-255-255 counter
+	call IncrementPlayTimeCounter
+	; check in-timer flag
+	ld hl, wReentrancyFlag
+	bit IN_TIMER, [hl]
+	jr nz, .done
+	set IN_TIMER, [hl]
+	ldh a, [hBankROM]
+	push af
+	ld a, BANK(SoundTimerHandler)
+	call BankswitchROM
+	call SoundTimerHandler
+	pop af
+	call BankswitchROM
+	; clear in-timer flag
+	ld hl, wReentrancyFlag
+	res IN_TIMER, [hl]
+.done
+	pop bc
+	pop de
+	pop hl
+	pop af
+	reti
+
+; increment play time counter by a tick
+IncrementPlayTimeCounter: ; 021c (0:021c)
+	ld a, [wPlayTimeCounterEnable]
+	or a
+	ret z
+	ld hl, wPlayTimeCounter
+	inc [hl]
+	ld a, [hl]
+	cp 60
+	ret c
+	ld [hl], $0
+	inc hl
+	inc [hl]
+	ld a, [hl]
+	cp 60
+	ret c
+	ld [hl], $0
+	inc hl
+	inc [hl]
+	ld a, [hl]
+	cp 60
+	ret c
+	ld [hl], $0
+	inc hl
+	inc [hl]
+	ret nz
+	inc hl
+	inc [hl]
+	ret
+
+; setup timer to 16384/68 ≈ 240.94 Hz
+SetupTimer: ; 0241 (0:0241)
+	ld b, -68 ; Value for Normal Speed
+	call CheckForCGB
+	jr c, .set_timer
+	ldh a, [rKEY1]
+	and $80
+	jr z, .set_timer
+	ld b, $100 - 2 * 68 ; Value for CGB Double Speed
+.set_timer
+	ld a, b
+	ldh [rTMA], a
+	ld a, TAC_16384_HZ
+	ldh [rTAC], a
+	ld a, TAC_START | TAC_16384_HZ
+	ldh [rTAC], a
+	ret
+
+; return carry if not CGB
+CheckForCGB: ; 025c (0:025c)
+	ld a, [wConsole]
+	cp CONSOLE_CGB
+	ret z
+	scf
+	ret